Using in_header: with anything other than true

[ando2040]

My configuration is showing me the last value from the array in the header (with in_header: true) - I want the first or second value (which should be close in time after now since this is a forecast) to be displayed in the header, what should I change?

in_header: raw gives "NaN"
in_header: after_now gives N/A,
in_header: true gives the last value in the array.

Where to go from this? The array data and config are below.

- entity: sensor.national_grid_renewables_3_day_forecast
    yaxis_id: MW
    color: black
    opacity: 0.8
    stroke_width: 2
    name: Nuclear + Solar + Wind
    data_generator: |
      var now = new Date().getTime();
      var forecastData = entity.attributes.forecast.filter(entry => {
        var startTime = new Date(entry.start_time).getTime();
        if (startTime > start){
          return true
        }
        return false
      });

      return forecastData.map(entry => {
        return [entry.start_time, entry.generation] 
      });
    type: line
    show:
      legend_value: false
      in_header: true

Sensor data: (real data is much longer but you get the idea, I want to show the next one in time not the last in the array, which is what I get with in_header: true)

forecast:
  - start_time: "2024-10-21T11:00:00+00:00"
    generation: 24751
  - start_time: "2024-10-21T11:30:00+00:00"
    generation: 24949
  - start_time: "2024-10-21T12:00:00+00:00"
    generation: 24900
  - start_time: "2024-10-21T12:30:00+00:00"
    generation: 24707
  - start_time: "2024-10-21T13:00:00+00:00"
    generation: 24448

Reason being: the next one in the forecast is quite close to reality, the last one in the forecast is very likely to have changed significantly by the time we get there...

I'll probably end up fudging this by creating a custom sensor to get the now value as a state rather than first value in an array, but it would be good to know how to achieve this, or if it's a feature/bug with after_now (the setting I was expecting to use).